Method for controlling some parameters in connection with manufacturing of plastic articles

ABSTRACT

A method for a supervising and/or controlling of discharged volume or weight of plastic mass flow for e.g. extrusion, injection moulding or blow moulding in a system including a plastification section, which feeds the plastic melt to a volumetric discharge device. This discharge device discharges in turn the molten material continuously or in portions to a forming tool. The pressure and temperature of the plastic melt is determined in connection with the discharge device and the density of the plastic material is calculated by means of these measured values.

The present invention relates to methods for manufacturing of plasticarticles, and more closely to such a method giving rise to plasticarticles having desired wall thickness, weight per meter and/or weightper piece.

The invention is primarily intended to be used in connection with theforming of plastic articles by extrusion, such as blown film and foilextrusion, cable extrusion plastic coating, pipe and rod extrusion, butthe invention may also, after modifying of the discharge section, beadapted to processes for blowing of bottles and injection moulding.

By the invention it has been possible to manufacture plastic articleshaving precise dimensions without use of instruments for measuring thethickness or for measuring some other dimension, or complicated andexpensive weighing equipment for batch weight as referred to in e.g. DAS1 964 386.

An essential problem in connection with the manufacturing of plasticarticles is to be able to maintain an even quality of the articles, andthen especially with regard to the wall thickness. The thicknessvariations occurring in connection with e.g. film and foilmanufacturing, are resulting in that nominal correct thickness varyabove as well as under said thickness. The result thereof is that oneeither accepts a product the thickness of which is incorrect, or one hasto manufacture a product being thicker than the nominal one in order tobe able to guarantee desired thickness. In the one case the quality isfaulty and in the other case the plastic consumption is unsatisfactoryhigh. The latter case also gives rise to not only increased costs forthe consumed plastic but also for heating, extrusion energy,transportation and so on.

To overcome the above problems thickness measuring instruments have beenused within the plastic industry, but these instruments have causedproblems due to often used additives such as pigments and similar, dueto variations of the temperature of the sensed article through thethickness thereof, and due to the fact that plastic temperaturevariations have appeared caused by conditions depending upon differenttimes of the year and/or product and manufacturing conditions.Accordingly, the result is that a great number of calibration constantshave been necessary to identify, which the operator then has had to takeinto consideration for the continued production. The result thereof isthat on-line measuring of the thickness generally is an unpractical andunreliable method which cannot be used successfully for manual orautomatic control of the manufacturing process.

Another attempt to get rid of said deficiencies is described in DAS 1964 386. This attempt includes the development of a weighing method forthe batch weight in accordance with which the material is supplied tothe extruder or corresponding machine is weighed, batch by batch orcontinuously, and for a set amount of weighed input material determinesmanufactured length or time of manufacture. Thereby, it is possible tocalculate weight per meter or production per time unit. The weight perlength or time unit is then determined by means of a level switchmounted in the lower portion of the extruder funnel or in the pipebetween the extruder funnel and the extruder. The closer the extruderand the less area at the place of measuring the better the accuracy ofthe weighing. However, this leads to the fact that a very small reserveamount of raw material is maintained before the extruder which inconnection with trouble in the delivery of raw material or in theweighing equipment for the input material gives rise to production stopand deficiencies in the manufactured product. Production stop means lossof time and reduced profitability and unnecessary deficiencies in themanufactured article and involve extra trouble for taking care of thesefaulty articles and increased costs.

In the conventional technique in use today for manufacturing of plasticarticles, an extruder is used comprising a rotary screw arranged withina cylinder, which screw is responsible for the processing andhomogenization of the plastic material as well as the discharge thereofto a tool or to a mould for forming the final product. In connectionwith extrusion, this takes place continuously while injection mouldingand in some cases blow moulding make use of a discontinuous discharging,preferably carried out by an axial movement of the screw in which casethe screw operates in a way similar to a piston pump.

The flights of the screw do not work with complete sealing against thesurrounding cylinder wall but a slight flow backwards of the materialtakes place in the gap between the wall and the flights of the screw,which gives rise to the fact that the pumping efficiency is effected onone hand by the viscosity and pressure of the melt and on the other handdepending upon the degree of wear of the screw. These facts give rise tovariations in the discharged amount per time unit, in turn resulting inincreased scrap level due to product variations and to bad raw materialefficiency.

The above problems are solved by the present invention by separatingplastification and discharge from each other.

Also in this case, the plastification section may comprise a screwrotating within a cylinder. However, as the plastification section doesnot need to be used to build up pressure for the subsequent extrusion,this section may be optimal in order to only carry out the processing ofthe plastic, i.e. the plastification and the homogenization. This leadsto a better efficiency with relation to this section and thereby onealso obtains energy savings. Also other types of plastification andhomogenization principles may be used such as kneading and/or meltingduring heat supply and stirring.

The discharging section is based upon a volumetric metering device, suchas a gear pump or a piston pump. The piston pump may be provided with adevice making an alteration of the displacement thereof possible, e.g.by changing the stroke length and/or diameter thereof.

The change of density of the plastic due to effect from pressure andtemperature is in accordance with the invention used for control of someparameters in connection with the manufacturing of plastic articles. Forexample, the discharge unit can be controlled by manually orautomatically adjusting the discharge unit to produce goods havingdesired cross section area or volume per piece. The density of the rawmaterial is stated in the material specification and may be used asinitial value, referred to herein as the "nominal density", and be usedin combination with measured pressure and temperature for calculation ofthe density at the place of measuring. The density of the additives arenormally very close to the density of the plastic and further,constitute so relatively small amounts that the additives do not affectthe density of the plastic itself in any degree that would effect thefinal result.

By measuring pressure and temperature on the suction side of thedischarge pump in connection with extrusion, it is possible to determinethe production in weight per time unit, preferably grams per minute, andtogether with measuring of the length, to determine the weight per meterand in connection with the injection moulding and blow moulding weightper manufactured unit. The measuring of pressure and temperature on thesuction side of the pump constitute in this connection two parametersfrom which it is possible to determine the density of the plastic massat the discharge from the pump and which, due to the fact that thevolume discharged from the pump is known by appointing either its r.p.m.or the total number of revolutions per discharge or the discharge of thepiston pump per cycle, can give the basis for determining theproduction. In this embodiment, the discharged volume per time unit orper discharging time is constant.

The density may also be calculated by controlling the process toconstant pressure and temperature on the suction side of the dischargepump. In this case, the density may be set as the calculated value ofthe density at the set points given for pressure and temperature.

By the above steps brought into practice, the objective is reached toarrive at accurate control of desired dimensions of the final productwithout obtaining unreliable safety in the production due to too smallreserve of material before the extruder, what might lead to immediateproduction stop if there are break downs in the material delivery orproblems in the input weighing system.

A man skilled in the art may modify the invention in different ways, butsuch modifications are intended to be within the scope of the inventionas defined in the attached claims.

I claim:
 1. Method of controlling process parameters to obtainsubstantially constant dimensions of articles of plastic materialmanufactured by extrusion, blow moulding or injection moulding in whicha volumetric discharge unit is used for discharge of a flow of plasticmass of said plastic material to a forming tool, comprising the steps ofdischarging said plastic mass at a constant volume per time unit or perdischarging time, measuring pressure and temperature of the plastic massflow at a position before the discharge unit, calculating the density ofthe plastic mass flow at said position to obtain a calculated value forthe density starting from nominal density of the plastic mass,establishing the weight of the articles manufactured per discharge timeor per time unit by multiplying the calculated density with thedischarged volume per discharge time or per time unit, and controllingprocess parameters based upon changes of the calculated density of theplastic mass flow to obtain said substantially constant dimensions ofsaid articles of said plastic material.
 2. Method in accordance withclaim 1, characterized by that in the controlling step the dischargeunit is adjusted so that the articles produced have a set desired weightper length unit or per article.
 3. Method in accordance with claim 1,characterized by that in the controlling step the discharge unit isadjusted so that the articles produced have a set desired crosssectional area or volume per article.
 4. Method in accordance with claim2, characterized by that in the controlling step the discharge unit isadjusted so that the articles produced have a set desired crosssectional area or volume per article. PG,10
 5. Method in accordance withclaim 2 wherein in the controlling step the discharge unit is adjustedmanually.
 6. Method in accordance with claim 2 wherein in thecontrolling step the discharge unit is adjusted automatically.
 7. Methodin accordance with claim 3 wherein in the controlling step the dischargeunit is adjusted manually.
 8. Method in accordance with claim 3 whereinin the controlling step the discharge unit is adjusted automatically. 9.Method in accordance with claim 4 wherein in the controlling step thedischarge unit is adjusted manually.
 10. Method in accordance with claim4 wherein in the controlling step the discharge unit is adjustedautomatically.